I'm trying to implement a linked-list ADT for my lab course. So far, I can't figure out what am I doing wrong. After successful compilation I try to add some more objects and nodes, but after inserting the second element (the first one is created just fine) into the second object and trying to link those two I get an exception message shown below:

I'll now provide all the necessary codes for the above, but as you can see, my showstructure code is wrong too as, it doesn't show the whole list at the same time, it just shows 1 element at a time, so I'd appreciate it, if anyone could help me to figure out my mistakes here. Thanks in advance.